The present invention relates to a method and an apparatus for recording a radiation image on a recording medium.
The invention more particularly relates to such a method and apparatus for use in the field of mammographic radiation image recording.
In mammography a radiation image obtained through X-ray exposure of a patient""s breast is commonly recorded in a dedicated mammographic image recording apparatus on a recording material such as a light-sensitive photographic film.
Such a dedicated image recording apparatus generally consists of a X-ray generator controlling the amount and the characteristics of the X-rays emitted by an X-ray source, a support for supporting the breast that it to be irradiated and a slide wherein a recording system, for example a cassette comprising a light-sensitive recording material and an intensifying screen, is positioned so that it is exposed to X-rays transmitted through the breast. The apparatus commonly also comprises an electronic signal processing unit by means of which different irradiation programs can be selected and by means of which the settings of the different parts of the apparatus are controlled and adjusted.
Mammographic images are taken when there is an instant need for diagnosis, e.g. when the patient is referred to the radiography department e.g. on request of a gynaecologist.
Apart from these occasions, mammographic images are frequently taken as a part of breast cancer screening programs.
Breast screening programs comprise the recording of a mammographic image of a person on repeated occasions, for example once a year. Part of the screening is a comparison of the mammographies taken in past years with a newly taken mammography in order to detect any changes in the breast tissue or in order to follow the evolution of suspicious elements which have been detected in the breast.
A prerequisite for optimal comparison of mammographies taken at different points of time e.g. as a part of such screening programs is that the image quality of the mammographic image is high, i.e. that a high resolution and high contrast image is obtained, and that this high quality remains constant over time. Even very small differences in the photographic characteristics of the mammographies may have an influence on the results of a comparison of mammographic images taken at different points in time, such differences may make such a comparison an unreliable basis for diagnosis.
This fact has been recognised by several international organisations among which the American College of Radiology: xe2x80x9cWidespread mammographic screening has the potential to significantly reduce mortality from breast cancer. However, the effectiveness and success of such screening and of all mammography depends on the consistent production of high resolution, high contrast mammographic imagesxe2x80x9d.
Several organisations have recommended the performance of quality assurance tests and have set up detailed manuals on the requirements of these tests.
Several governments have made the performance of quality assurance tests and the quality monitoring mandatory for breast screening programs to be accepted.
There is thus a constant aim among manufacturers of recording media and among manufacturers of recording equipment to enhance the consistency and the reproducibility of the image recording. As these companies are often not the same, it may be possible that the characteristics of the recording media, and more especially the occasional changes of these characteristics, are not fully known to the equipment manufacturer and can thus not be taken into account to an adequate extent.
In a radiographic image recording apparatus and more specifically in a mammographic image recording apparatus of the kind described higher, a large number of factors have an influence on the ultimate quality of the recorded image and on the consistency of the recording throughout time.
Among these factors the most important are:
the transfer function of the X-ray source, i.e. the spectrum of irradiation as a function of the set value of mAs, kV, anode characteristics, filtration of the X-ray source,
the conversion of the input X-ray spectrum into an output X-ray spectrum modulated by the object that is irradiated, in casu the patient""s breast,
the transfer function of the recording material,
In case the recording material is a photographic material this transfer function expresses the conversion of the X-ray image into a latent image.
the transfer function expressing the conversion of the latent image into a visible image.
In case the recording material is a photographic material or a photographic material which is accompanied by an intensifying phosphor screen, this material requires chemical development of the latent image to gain a visible image. This transfer function then expresses the characteristics of the development.
Although for the ease of explanation it is assumed hereinafter that the recording medium is a photographic material or a photographic material which is accompanied by an intensifying screen, other kinds of recording systems may be envisaged.
For example a photostimulable phosphor screen might also be a suitable recording material.
It is also possible to record the radiation image by means of a direct radiographic sensor such as a sensor comprising charge coupled devices or amorphous silicon flat panel detectors or the like.
It will be clear that in case the latter kinds of recording systems are used, chemical processing and corresponding adjustments are not required.
The transfer function of the X-ray source is known to the manufacturer of X-ray image recording equipment.
When setting the radiographic recording apparatus the expected characteristics of the X-ray modulation by the object that is irradiated is taken into account through the use of a model-description which is stored in the electronic signal processing unit controlling the setting of the apparatus or through intervention of the X-ray technologist having the knowledge to do so.
The effect of other very important factors however, namely the transfer function of the recording material and the characteristics of the conversion of a latent image into a visible image (which in the case of a photographic recording material are the characteristics of the chemical processing), is generally not completely known to the manufacturer of the radiographic image recording equipment.
These effects may vary to a certain extent due to batch-to-batch changes of the recording material, day-to-day variations of the processing conditions due to temperature variations, humidity, decay of the chemical properties of the processing bath, non-uniform characteristics of the intensifying screens used with a photographic film, X-ray absorption by a cassette conveying a radiographic film or intensifying screen-radiographic film combination, etc.
To a large extent these effects are taken into account by performing a calibration of the apparatus. During this calibration characteristics of the recording material as well as characteristics of the developing process (in case the recording material is a radiographic film) are determined and the recording equipment is adjusted taking into account these characteristics.
Nowadays these calibrations are generally performed by a service technician on a regular basis. However, since this kind of intervention by a service technician is expensive, the frequency of these interventions is kept rather low. Several causes of inconsistency such as day-to-day variations of the developer conditions, batch-to-batch film variations etc. occurring in between succesive interventions of the service technician cannot be solved.
Consequentially it is possible that in between two successive interventions the image quality of the radiation images becomes inconsistent.
EP 432 722 relates to an apparatus for writing information on X-ray films. More particulary means are provided for identifying a type of film, e.g. tagged with a mark S such as a notch that has been cut in each film. An exposure correction means performs corection to determine an amount of exposure that is appropriate for a particular film type. Using the determined amount of exposure, a printer is controlled to write patient identification data on the X-ray film.
EP 432 119 discloses a method for X-ray diagnosis in particular for mammography, wherein the exposure of a film or the like is regulated to an optimal level. The radiation that has passed through an object is measured by means of an arrangement of detectors and on the basis of the result of this measurement the time of exposure is regulated.
It is an object of the present invention to provide a method and an apparatus for recording a radiation image, more specifically a mammographic image, that provides consistent image quality throughout time.
It is a further object of this invention to provide such a method and apparatus without the need for augmenting the frequency of the interventions of a service technician.
Still further objects will become apparent from the description here below.
The above mentioned objects are realised by a method having the specific features defined in claim 1.
The method of the present invention is advantageous in that a high degree of consistency is obtained between radiographic images taken at different points in time. The uniform quality is obtained without the need of frequent intervention by a service technician. The higher degree of consistency can thus be obtained at a much lower cost.
The method of the present invention comprises a first step wherein a recording medium which is actually used in the image recording apparatus is identified.
By the term xe2x80x98recording mediumxe2x80x99 is meant in the context of this invention the medium itself, for example a specific photographic film, a specific photostimulable phosphor screen, an X-ray sensitive solid state sensor or the like. The term is also meant to cover combinations that together form a recording system such as a photographic film xe2x80x94intensifying screen combination or a cassette conveying such a combination.
Identification in the context of the present invention means determining a unique identification item that is associated with the recording medium, such as an identification number, which has a one to one relation with the recording medium that is used in the X-ray recording apparatus during the ongoing X-ray recording.
An identification item can be provided on the recording medium itself or on a wrapper or cassette conveying the recording medium.
A large variety of identification items are suitable in the context of the present invention.
In one embodiment identification consists of reading a bar code which is for example provided on a cassette conveying a radiographic film and occasionally also an intensifying screen.
In another embodiment identification consists of reading an identifier, such as an identification number from an electronic memory (e.g. an EEPROM) provided on the cassette conveying a recording medium, for example an X-ray film and (an) intensifying screen(s) or a photostimulable phosphor screen.
In still another embodiment identification can consist of entering an identifier via keyboard or via other interfacing means into the recording apparatus.
Still other alternatives are possible such as infrared identification, electromagnetic identification, optical identification, mechanical identification etc.
Next, pre-stored characteristics regarding an identified recording medium are retrieved. These characteristics are for example the type of the radiographic film, the age of the radiographic film, the batch number of the film, characteristics of a certain batch number, the type of intensifying screen, attenuation characteristics of a cassette conveying a screen-film combination, the date of production of the film, etc.
Next, the adjustments of the X-ray source are set taking into account the characteristics of the individual recording medium that is actually identified.
In this way the adjustment can be set for each individual image recording taking into account the specific properties of the recording medium that is used for that specific image recording.
In case the recording medium is a radiation sensitive film or another recording medium that needs to be subjected to processing in order to render the latent radiation image visible, it is further advantageous to take into account the deviation of the applicable processing conditions from expected processing conditions when setting the adjustments of the X-ray source. This is advantagous since processing conditions may vary with time due to chemical decay or bath exhaustion, environmental changes such as temperature changes etc. Such changes of processing conditions have an effect on the image quality of the recorded radiographic image.
In the context of the present invention the words xe2x80x98actual processing conditionsxe2x80x99 are to be interpreted as the instant processing conditions applicable at the time of image recording or processing conditions applicable within a time interval between successive calibrations.
Consequentially the data regarding to processing have to be communicated to the recording apparatus so that these conditions can be taken into account when setting the adjustment of the X-ray source.
The properties of the applicable processing can for example be determined by exposing a test film under controlled exposure conditions in the radiation image recording apparatus, developing the exposed test film in the processing bath, measuring characteristic parameters such as contrast and sensitivity on the developed test film and communicating the measured values to the control electronics of the image recording apparatus.
Communication of the processing properties to the recording apparatus can be performed via an electronical connection of a measuring device and the recording apparatus.
For example an exposed and developed test wedge can be measured by means of a densitometer the output of which can be fed via a cable connection to the recording apparatus. The densitometer may be a separate apparatus or may be part of the processor.
Alternatives such as optical data transmission, infrared data transmission, radiowave transmission or manual entering via keyboard etc. are also possible.
Once the recording material has been identified and the processing conditions, if applicable, have been communicated to the recording material the source of radiation is adjusted taking into account these data and a radiation image of the object on the recording medium.
The invention further discloses an apparatus as defined in claims 7-11.
Specific features for preferred embodiments of the invention are disclosed in the dependent claims.
Further advantages and embodiments of the present invention will become apparent from the following description and drawings.